This invention relates to the determination of polymer characteristics. In practice, polymer is typically melt processed in onward production or xe2x80x98conversionxe2x80x99xe2x80x94and so measurement of Melt Flow Index (MFI) is of particularly relevance.
Key polymer properties, such as tensile strength, solubility, impact resistance and melt viscosity, are associated with (average) molecular weight. For a given polymer, a higher molecular weight generally affords greater resistance to impact. On the other hand, lower molecular weight polymers are useful for thin films or filaments.
Polymer conversion requires close control of melt viscosityxe2x80x94which is related to molecular weight, so that:
the polymer xe2x80x98melt viscosityxe2x80x99 (or melt flow indexxe2x80x94MFI) is suitable for the conversion process (such as moulding or extrusion); and
a polymer product has desired physical characteristics.
To address this, most polymers are available in a range of average molecular weight grades. In practice, these grades are specified in terms of melt viscosity (MFI)xe2x80x94for example to 5% tolerance of target specification.
Melt viscosity is prescribed under a standard test regime, intended as xe2x80x98staticxe2x80x99 and stablexe2x80x94and is routinely measured using a so-called xe2x80x98manual graderxe2x80x99, as specified in ASTM D 1238 standard; being xe2x80x98reportedxe2x80x99 as xe2x80x98melt flow indexxe2x80x99 (MFI).
However, the standard test is not readily achieved, or directly usable, for production control. The Applicants have devised an improved xe2x80x98dynamicxe2x80x99 test regime, which offers improved consistency and responsivenessxe2x80x94and admits the possibility of MFI as a polymer production control factor. Essentially, control of MFI requires control of average molecular weight during polymerisation.
In polymer production, rapid and reliable MFI measurement allows a more precise assessment of a polymerisation (reactor) output and so enables effective reactor process controlxe2x80x94ultimately on-line and in real-time.
A polymerisation reaction typically generates polymer in a powder form, which is highly reactive and difficult to handle. This polymer powder is therefore commonly converted into a more stable, and xe2x80x98user-friendlyxe2x80x99, granular or pelletised form, for collection, storage and onward transport to customer end users and re-processors.
Pelletisation or granulisation is typically undertaken in an extruder, as is any downstream (mechanical) mixing, blending and re-processing. Precise MFI knowledge during extrusion in such conversion and onward re-processing is just as relevant as in original polymerisation.
(MFI) viscometry for xe2x80x98rapid-responsexe2x80x99 determination of polymerisation reaction output;
(on-line) reactor control through (MFI) viscometry; and
post reaction powder-granule conversion and re-processor blending.
Polymers are commonly produced,ultimately as a fine powder particulate, by continuous catalytic polymerisation, initially in a gas and/or slurry phase.
Polymerisation is influenced, inter alia, by reaction time, temperature, pressure, monomer, catalyst and impurities concentrations in the reaction mixture.
A limited range of factors is available for such catalytic reaction control. Primarily, control of molecular weight is achieved through inhibition of chain growth. In turn, chain growth inhibition involves addition of a monomer inhibitor agent (a so-called Terminating Agent, Chain Transfer Agent or CTA), such as hydrogen.
More particularly, in (continuous) xe2x80x98catalysedxe2x80x99 polymerisation, to a target MFI specification, reaction is controlled by either:
the addition of a xe2x80x98chain terminatorxe2x80x99, which reacts with a catalyst to stop polymerisation; or, more usually,
the introduction of a xe2x80x98chain transfer agentxe2x80x99 (CTA), which reacts with the growing polymer chain, to prevent further polymerisationxe2x80x94but leaves the catalyst able to start polymerisation of a new chain.
Thus, both MFI determination and hydrogen (inhibitor) input control can be used constructively to control molecular weight.
Control of polymer chain growth is a xe2x80x98delicatexe2x80x99 process, with a need to relate reactor chain transfer agent concentration to molecular weight. The relationship is process dependantxe2x80x94taking account of individual reactor and catalyst characteristics.
W09324533, EP002747 and U.S. Pat. No. 3,356,667 variously rely upon monitoring, and adjustment, of diverse reaction factors, such as temperature, pressure, CTA and/or monomer concentrationxe2x80x94as a basis for computation and control of output polymer MFI. W09641822 adopts a predominantly mathematical approachxe2x80x94but a significant time correction, between sampling and MFI result, must be factored into the analysis.
None of this art addresses direct, on-line, viscometric measurement of polymer output MFI, as a continuous on-line control factorxe2x80x94which in turn justifies special visometric methodology and apparatus. Conventional viscometry is time-consuming, and performed off-line, commonly in laboratory controlled conditions.
Formative steps towards closed-loop control have been taken with the emergence of more reliable MFI measurement technology, such as the Applicants"" own earlier work. An example is the Applicants"" (model P5) viscometer and attendant interpolative (graphical) measurement techniquexe2x80x94as taught in their UK Patent No. 2210466.
A polymerisation reaction (process) control aspect of the present invention places even greater reliance, upon responsive MFI measurementxe2x80x94and embraces refinements to the Applicants"" (model P5) viscometer technology and attendant measurement technique.
In practice, it can be very difficult to maintain completely steady conditions in the reactor. Disturbances in changing grades to the are inevitablexe2x80x94with attendant MFI xe2x80x98driftxe2x80x99 from target specification. Frequent MFI measurements on polymer from the reactor guide adjustments in the flow of chain transfer agent, to correct any such drift.
Failure to compensate adequately for disturbances results in material properties outside prescribed MFI limits, and so one which must be sold at a substantial discount compared to xe2x80x98first (or consistently to specification) gradexe2x80x99 polymer.
These xe2x80x98downgradingxe2x80x99 (or off-specification) losses can be reduced by post-reaction blending of product and restricting the frequency of grade changesxe2x80x94but the capital equipment and attendant running costs are high and there are limits to the xe2x80x98spreadxe2x80x99 of MFI""s which can be dealt with in this way.
The number of xe2x80x98gradexe2x80x99 changes can be reduced by making larger-quantities of each grade at longer intervals, but the cost of the higher stock levels needed eventually outweighs the benefits. Despite this, between some 5-15% of product of conventional process control regimes is commonly downgraded by poor MFI control. Even xe2x80x98goodxe2x80x99 product produced by this approach has a large xe2x80x98scatterxe2x80x99 of MFI.
In a typical case, the chance of a customer end user or re-processor receiving two successive batches at opposite extremes of the MFI range could be 1 in 26, rather than 1 in 1600 or more if effective MFI control were used.
Such poor performance is the result of inadequate, or inaccurate, understanding of the relationship between MFI and concentration (or flow) of chain transfer agents (or terminators). This applies to both xe2x80x98steady statexe2x80x99 and dynamic conditions.
The relationship is often totally obscured by a series of system errors, such as:
Inaccurate measurement of MFI. Studies of the manual method by the ASTM (standards body), allied to industrial experience, have shown that a standard deviation of 5% in the middle of the operating range, rising to 15% at the upper and lower limits, can be expected for a single (given) reading.
Although other means of xe2x80x98measuringxe2x80x99 MFI exist which give lower scatter, the relationship between their values and MFI are neither simple nor consistent. This introduces an equally serious error.
Unrepresentative samples of reactor product.
In many cases the sampling device is incorrectly designed and/or situated, so that a sample drawn is not representative of reactor contents.
Variation in pre-conditioning required to remove monomer and active catalyst.
Degradation of the polymer before or during measurement.
Ex-reactor polymer often contains catalyst, which will react during melting and alter MFI, unless de-activated. The polymer also contains no anti-oxidants and will be chemically degraded if melted in a conventional extruder. If the polymer has been exposed to air (with its oxygen component), this degradation will be much more severe. Existing methods of treating the polymer to avoid these problems during manual measurement are too slow.
Delay between events in the reactor and MFI measurement.
In hitherto known practice, a large deviation in MFI has to occur before a decision to apply a correction could be made. This adds xe2x80x98dead timexe2x80x99 to that already present in the reaction system and often makes control almost impossible.
Other problems of a xe2x80x98manualxe2x80x99 grading technique are measurement times varying from some 20 to 50 minutes (depending on MFI), and delays entailed in sample collection and pre-treatment and cleaning of apparatus, with attendant costs.
Other sources of delay are placement of the sampler downstream of the reactor, the time required to deactivate the catalyst and/or remove monomers and the time to take the MFI measurement. Using manual methods, the last two alone will typically give a dead time elapse of some 30-80 minutes (depending on MFI), before any MFI deviation is detected. Typically, the combined effect of these factors results in the aforementioned xe2x80x98downgradingxe2x80x99 of 5 to 15% polymer produced.
Inaccurate measurement of concentration of the chain terminator or transfer agent.
Typical causes are incorrect choice or poor maintenance of the measuring instrument, and poor position of the sampling point, giving unrepresentative samples or physical changes (eg condensation) between sampling and measurement. In some cases concentration cannot be measured and flow into the reactor must be used. As the reactor input/output flows involved are relatively small, poor instrument choice or maintenance will give serious errors.
If the errors and delays attendant the foregoing contributory factors are substantially reduced, the more meaningful (measurement) data obtained opens up the prospect of closed-loop control of MFI in continuous catalysed polymerisation.
In principle, accurate measurement of flow and/or concentration of chain transfer or terminating agents requires only the application of known techniques. However, it has not hitherto been practical to overcome the aforementioned problems of MFI inaccuracy, representative sampling, sample degradation and measurement delay.
According to one aspect of the invention, a method of closed-loop regulation and/or control, of polymerisation, in a reactor (11), using a chain transfer agent (CTA), to determine average polymer chain length; the method comprising the steps of: periodic selection, isolation and conditioning of discrete samples of reactor polymer output; direct, on-line, viscometric melt flow index (MFI) sample measurement; comparison between a directly measured sample MFI value and a desired or target reactor polymer output MFI value; periodic adjustment of chain transfer agent (CTA) supply to the reactor, according to ongoing sample MFI determination.
Preferably, successive samples of reactor polymer output are individually isolated, conditioned and stabilised in a sampler (50), passed through a cyclone separator (12), into an accumulator (14), and then to an a MFI test viscometer (15), configured for repeated, isolated sample MFI determination.
Such polymerisation control methodology, may include the step of sample deactivation, through the introduction of a catalyst inhibiting agent, such as isopropanol, prior to sample viscometric measurement.
According to another aspect of the invention, polymerisation reactor control apparatus, for the above method of polymerisation reaction control, include: a CTA supply (21), a CTA flow meter (23), a CTA flow regulator (22), fitted between the CTA supply (21) and reactor (11), a CTA flow comparator (24), a CTA concentration set point unit (29/24), a CTA concentration controller (28), an MFI comparator (27/28), an MFI set point unit (31), a viscometer (15), for sample MFI determination, the reactor CTA concentration being controlled by the CTA flow regulator, in accordance with sample MFI determination by the viscometer.
Desirably, the CTA flow regulator (22) comprises a valve, controlled by a flow control signal (34), derived by the CTA flow comparator (24), from a CTA flow set-point control output (43/42), based upon the viscometer MFI determination, and actual CTA flow input (44),derived from the CTA flow meter (23).
Conveniently, a measurement output (38) from the viscometer (15), is applied to an MFI comparator (27/28), together with a reference signal (39), from the MFI (target) set-point unit (31), to produce an MFI correction factor, for the CTA concentration controller (28), which issues a set command (42), taken into account by a CTA flow comparator-controller (24), to generate the flow control output (34).
Preferably, the concentration of CTA in the reactor (11), is monitored, through a tapping (36),with a sensing (gas) chromatograph (19), the chromatograph sensing output (37) being applied as an input to the CTA concentration controller (28).
In practice, successive reactor output samples are individually isolated, stabilised and pressure-conditioned,upon transfer from the reactor to the cyclone separator (12), via the sampler (50).
Nitrogen gas (N2) may be employed as a sample transfer medium,between reactor and sample MFI determination viscometer.
In a particular example,polymerisation reactor control apparatus, includes a cyclone separator (12), a funnel (63), a ram compaction chamber (71), connected to the funnel, a compaction ram (73), movable within the compaction chamber, a melter (74) connected to the compaction chamber, a gear pump (77), connected to the melter output, a measurement die block (81), connected to the gear pump output; wherein MFI determination is undertaken upon sample material from the cyclone separator,once collected, mixed and temperature pre-conditioned in the funnel, discharged into the compaction chamber, and consolidated upon the melter by the compaction ram, whereupon a conditioned sample melt stream (80) is delivered by the gear pump, to the die block for MFI test flow measurement.
Another aspect of the invention provides a sample isolator, for the polymerisation reactor control apparatus, as claimed in any of claims 4 through 10, including an isolation chamber, for temporary storage of a sample of polymer output; a multiple port rotary valve, with a rotary valve member, having a diametral passage, serving as a sample conditioning and transfer chamber, and selectively alignable with circumferential ports, connected to transfer, conditioning, discharge and flushing media; whereby, upon receiving polymer sample from the isolation chamber, the chamber is connected successively with respective ports, to effect sample conditioning, including deactivation and addition of transfer medium, [prior to] discharge for viscometric determination; further port connection effecting vacuum flushing, to purge remaining polymer, and valve cooling, following sample discharge.
Yet another aspect of the invention provides a viscometer for MFI determination, for the polymerisation control method or apparatus, a rotary diverter valve (82), in a die block (81), incorporating a plurality of relatively angled capillaries, for output polymer sample diversion, to a selected one of multiple measurement dies (91, 93), subject to differential respective pressure heads (Px, Py), for MFI viscometric test conditioning, without interruption for measurement head change.
A linear reciprocating shuttle valve may be used to selectively divert a polymer sample, to a viscometer, or waste discharge.
According to a further aspect of the invention, a polymer (reactor/reaction) process control method, comprises the steps of: applying a chain transfer agent (CTA) to a reactor, to control a polymerisation reaction within the reactor, measuring the melt flow index (MFI) of a xe2x80x98(pre-)conditionedxe2x80x99 sample of reactor polymer output, determining and applying a CTA concentration correction, according to a preceding sample MFI determination, and measuring the effect upon reactor (sampled) output MFI; continuing successive output sampling MFI measurement and attendant CTA (flow) adjustment, to stabilise reactor output to desired MFI target criteria.
The invention also provides a polymerisation process control method of reactor polymer output MFI, by selection, conditioning and measurement determination, of successive reactor samples, isolated from the reactor (environment)and purged or neutralised of CTA, to suppress ongoing reactivity and promote sample stability and representation of reactor polymer output; sample MFI determination being conducted after initially flushing the sample, with a neutralising, and anti-oxidising, agent; such MFI sample determination being effected continually/periodically, to update/adjust CTA flow and thus reaction conditions and polymer output, so that the polymer output characteristics are pre-determined.
The invention also embraces a method of control of melt flow index (MFI) of polymer output, (in a continuous, catalysed, gas and/or slurry-phase, polymerisation reaction process), by use of a chain transfer agent (CTA), such as hydrogen,the method including the steps of: measuring CTA concentration, by conducting gas chromatography on reactor process environment, measurement of actual CTA flow to the reactor, MFI determination of a selected discrete sample of reactor polymer output, comparison of sample MFI with a target MFI set point, generation, from the MFI comparison, of a target CTA concentration set point, comparison of actual CTA concentration, with the CTA concentration set point, adjustment of CTA concentration in response to actual CTA concentration, actual CTA flow, and CTA concentration set point, by adjusting CTA flow to the reactor.
Desirably, a method of polymer process control, includes taking samples to a set target size, in the size range from 80 to 120 gms, to a tolerance of not more than 5%; freeing samples substantially of monomer and active catalyst, to inhibit changes in MFI during subsequent sample MFI determination, and reducing fire or toxic hazards, keeping the total time for sampling, transfer and de-activation to less than the cycle time of a viscometer undertaking sample MFI determination.
Preferably, an MFI process control method, includes the step of suppressing sample degradation, before sample MFI determination, by trickling isopropanol vapour, from a boiler generator, into an air purge, both as an anti-oxidant, and to prevent sample contamination, by the presence of oxidisation.
Time Lag
Although catalytic polymerisation reactors may not have an obvious product xe2x80x98transportation delayxe2x80x99, many of them exhibit a xe2x80x98dead timexe2x80x99in the relationship between MFI and (CTA), or flow of CTA, and which is of the same magnitude as the first order lag time and is related to the polymerisation rate.
In practice, resolution of this requires a facility, encompassed by preferred embodiments of the invention, for anticipation of a first order time lag, related to the polymerisation rate of xe2x80x98continuous stirred tankxe2x80x99 reactors;
In these circumstances, the control algorithm employed by preferred embodiments of the invention reflects the significant dead times and first order dynamics exhibited by polymerisation reactions. This requires considerable modification of, or departure from, conventional dead time process control algorithms.
In practice, the Applicants have found that the interval between samples is related to the interval between reactor discharge xe2x80x98pulsesxe2x80x99 xe2x80x94which is in turn linked to polymer xe2x80x98make-ratexe2x80x99 and xe2x80x98bed levelxe2x80x99 within the reactor.
Thus the assumption of fixed measurement intervals inherent in conventional process control schemes is invalid, especially where a major disturbance is present.
Rather, in preferred embodiments of the invention, statistical techniques are employed to update the process model and evaluate the reality and magnitude of the predictions obtained from it.
If a sample remains reactive, it is no longer representative of reactor contents. Hence the provision, in preferred embodiments of the invention of sample deactivation.
Thus, for example, polymer is exposed to 10 to 13% oxygen, at 80 to 110 C., for 2 to 4 minutes, to deactivate second-generation, so-called xe2x80x98Ziegler-Nattaxe2x80x99 catalysts;
Catalyst retention would otherwise promote ongoing reaction and change in MFI from that characterising reactor conditions.
Generally, analogue controllers are unsuitable, but in preferred embodiments of the invention, digital hardware, using model-based control, such as the so-called Dahlin algorithm, is employed. The software can be incorporated in the Applicants"" (model P5) viscometer.
Overall, reactor sampling for MFI determination and polymerisation control according to preferred embodiments of the invention must satisfy the following requirements:
a the sample is representative of the reactor contents;
sampling delay is less than 1.5 minutes from sample request;
sample size is set between 80 and 120 gmsxe2x80x94desirably for consumption within a single measurement cyclexe2x80x94and does not vary from the set value by more than 5%, to avoid sample segregation;
samples are free of monomer and active catalyst, such that no fire or toxic hazard is presented and no changes in MFI take place during MFI measurement;
the total time for sampling, transfer and de-activation is less than the cycle time for the (say Applicants"" model P5) viscometer; and
the catalyst is treated chemically, so that it will not react with the polymer during measurement to alter MFI.
In effectively addressing such process criteriaxe2x80x94in conjunction with MFI viscometry refinement, discussed laterxe2x80x94preferred embodiments of the present invention make closed loop control of MFI a practicable proposition.
Hitherto, reliance upon xe2x80x98standardxe2x80x99 MFI measurements has contributed to poor MFI control in polymerisation.
Over the last some 20 years, viscometer designs have sought to improve accuracy and/or speed of MFI measurement.
Some retain a constant pressure approach, reflecting the standard MFI laboratory test conditions, but use a variable speed pump with a continuous polymer feed.
However, such units can be no taster than a xe2x80x98manual graderxe2x80x99, as the viscoelastic polymer melts are slow to respond to a change in flow rate.
Other designs use different mechanical arrangements to derive melt viscosity data, but their data is not simply and predictably related to the xe2x80x98manualxe2x80x99 MFI valuesxe2x80x94which (despite their limitations) remain an industry standard.
Many continuous flow viscometers have relatively large internal volumes and small throughput, so that their readings are delayed for long and variable times.
Most use a screw extruder to melt the polymer prior to measurement.
As polymer samples from the reactor have no anti-oxidant present, oxygen entrained by the screw attacks the chains during melting, reducing average chain length. Large, unpredictable errors in MFI are the result.
As a precursor to MFI measurement, the use of a ram for (sample) melting, in preferred embodiments of the invention, substantially removes air or other deactivating agent from the sample before melting commences, so degradation of (polymer) chain length from this cause can be avoided.
Viscometer design and construction factors become particularly acute with the constructive use of MFI in reactor process control for (catalytic) production of polymerxe2x80x94which is contingent upon prompt and reliable xe2x80x98real timexe2x80x99 measurement of melt flow index.
An objective is the use of MFI as an active feedback (correction) factor, in on-line, closed-loop (polymerisation) process controlxe2x80x94to produce polymer to prescribed standards or characteristics.
Generally, xe2x80x98squeezing or squashingxe2x80x99 polymer through a (measurement or process) die, in conventional viscometry, folds up the polymer chain and distorts its characteristics, impeding a satisfactory melt flow index determination.
The Applicants have evolved a measurement regime of melt index determination by a graphical interpolation techniquexe2x80x94the basis for which is taught in the Applicants"" UK Patent No. 2,210,466, as utilised in their model P5 viscometer. The results of the Applicants"" MFI test conform closely to the standard test.
More specifically, reflecting MFI measurement precision, in a plot of Log MFI readings from the Applicants"" model P5 viscometer, against Log MFI of readings from the standard laboratory or manual test, the xe2x80x98mutual calibrationxe2x80x99 curve departs somewhat from an xe2x80x98idealxe2x80x99 45 degree or 1:1 ramp ratio sloping line, to a slight, but consistent, departure slope of 1.0163, for a granular product.
This useful xe2x80x98conformityxe2x80x99 arises from a low shear rate measurement attendant relatively slow flow. Thus, in the Applicants"" viscometery technique, the polymer xe2x80x98structurexe2x80x99 under test is not extended too far. This same shear rate is consistent, over a large range, with molecular weightxe2x80x94otherwise the test would only reflect visco-elasticity or xe2x80x98stickinessxe2x80x99 and viscous drag.
Through sample de-activation, such as with polar fluids, a substantially parallel (allowing for small scatter) comparative plot with a standard test is achieved, with a small offset. With improved reactor sample de-activation, such as with isopropanol, prior to testing, an almost coincident plot is forseeable.
Viscometry aspects of the present invention address improvements in the Applicants"" model P5 viscometer, enabling closed loop polymer reactor process control using MFI as a variable control factor.
Thus, for example, enhancements to critical viscometer constructional features include:
high throughput; with a high force piston, high capacity (eg 1.186 cc) gear pump and careful melter configuration, to achieve high flow rates of molten (polymer) sample product, of uniform output stream temperature;
careful matching of the shapes and sizes of certain key components and attendant operational forces; such as in particular:
pressure at the melter entrance;
actual melter size/capacity;
melter top, gap and tail cone shape/profile; and
pump sizing/capacity, relative to melter (surface) area;
singly and in co-operative combination.
uniformity of molten (polymer) product, by careful use of flow passage tapers and transitions, without the use of large volume and long passageways, which produce large delays and product back-mixing;
This strategy applies to all the viscometer xe2x80x98wettedxe2x80x99 parts, in the passage of a polymer sample through the viscometer.
low back-mixing in the (gear) pump, with pump swept volume/dead volume ratio optimised for the throughput used;
low frictional heating in the polymer, with a (gear) pump construction employing relief grooves at the delivery side and a xe2x80x98gentlexe2x80x99 tooth profile;
critical flow path capillary shape profiling;
Thus a combination of offset tangential feed to the capillary entry chamber, with a shallow (polymer dependentxe2x80x94but, say, 118 degree) included cone angle to capillary and a (say, 5 mm) radius between cone angle and entry chamber cylinder walls contribute to very low turbulence and back mixing in the capillary entry chamber.
Such cone angles and edge transition radii admit of variation and are not restricted to a simple cone angle, but embrace more complex forms, such as those illustrated in FIG. 5C.
These shapes make significant contributions to the overall response time of the viscometer, when changing from one material to another. The design is considerably faster than a chamber having no radius or no cone or no tangential entry.
The entrance effect is close to the entrance effect of the stepped configuration, because the flow of polymer fluid naturally follows close to the above shape when the flow becomes fully established, with a small recirculation component filling the corner of a stepped entry, which is polymer dependant.
Coning and radiusing eliminate the re-circulation phenomena and give virtually zero back-mixing. This in turn produces very steady flow conditions, with a high threshold for the (adverse) flow phenomenum known as melt fracture.
Steady flow contributes to a highly repeatable measurement, because the contributions of elastic and transitory components are minimised. The system settles to steady flow faster than with a series of harsh transitions and corners in the melt passageways. The cone entry effect is erroneously computed using standard equations for steady flow.
adoption, in some variants, of multiple, in particular dual or triple, capillary chambers, addressed by a single pump stream;
Such an arrangement is suitable for discrete charges of material, where there is a practical restriction on flow rate.
In a particular construction, the dual chamber has a plug valve to divert flow at a gear pump exit. This plug valve has a rotary valve member relying upon a plane ground shaft in a honed bore to effect sealing against the escape of test sample polymer. The valve is rotated by a pneumatic rotary actuator, using end stops of the actuator to obtain, 0, 90 and 180 degree (angular rotational) positions.
In one valve position, the flow is directed to one of the test chambers through an angled gallery in the valve member shaft itself. A corresponding arrangement prevails in another chamber, when the shaft is rotated to a second position. Small misalignments are allowed for by the use of ball-ended keys on the shaft ends.
Overall, this arrangement has a low volume and simplicity and avoids the need for a dual pump or planetary pump, which would have to run at a higher melting rate, (usually twice the rate used for a single pump), thus saving on the required melting rate of the system. This in turn allows a variable flow rate and use of dissimilar capillaries.
In particular, the following capillary operating combinations are available:
zero length capillary and long capillary having the same entrance geometriesxe2x80x94for extensional viscosity calculations on filming polymers;
two capillaries of differing diameters, but of same entrance geometries and same L/D ratiosxe2x80x94for multi-weight MFI determinations over very large product ranges;
two capillaries of same diameters and entry geometry, but differing lengths, to obtain sample shear stress/shear rate information.
an alternative, dual capillary chamber arrangement addresses a dual pump stream, and as such is suitable for discrete charges of material, where there is no restriction in flow rate, as would be the case with an extruder-mounted rheometer;
Such a double pump is appropriate when rapid changes in polymer properties have to be monitored, as for reaction extrusion requiring a comprehensive rheological test on the passing materials.
As to a dual shear rate measurement cycle itself, improvements include:
range extension on each capillaryxe2x80x94achieved by selection of two flow rates which straddle MFI test pressure for each material each time;
inclusion of a purge rate in each cycle, the purge having four functions; viz:
1. high clear-out ratio of old material from the measurement chamber;
2. balancing of the total quantity of material used in each test cycle;
3. total consumption of the quantity obtained in a discrete sample in each discrete test cycle;
4. balancing of response time, as defined by sampling to measurement time, by using a variable purge rate to compensate for variable flow rate, so that the amount of material consumed in each cycle is constant.
Generally, high flow rates have a better scour effect than low flow rates. There comes a point with each material where the flow rate achieves a scour that is independent of the direction of changexe2x80x94ie the hard to soft material transition is normally slower than the soft to hard transition.
These flow rates can only be achieved, according to preferred embodiments of the invention, when the passageways have smooth transitions as in the Applicants"" (models P3 and P5) viscometer geometries and the gear pump has a sufficient volume throughput to achieve these flow rates.
Rough transitions, particularly in the capillary entrance, and low volume pumps suffer from limitations due to the onset of melt fracture, which occurs before a sufficiently high purge rate has been achieved.
The choice of gear pump size at 1.186 cc/rev gives near optimum conditions, which would not, in the configuration used, be achieved with other pump sizes, such as 0.2 cc/rev or 0.584 cc/rev, commonly employed in conventional viscometers.
The Applicants have observed, with viscometer embodiments of the invention, the rapid clear-out phenomena in the region of 40 g/minute and 120 g/minute on various polymers. 120 g/min is close to the upper limit of the melt delivery of the liquid or solid fed rheometers.
In practice, with preferred embodiments of viscometers according to the invention, a purge is undertaken at the beginning of the cycle to clear old materials to the new. An estimation of the correct first flow rate of the MFI test is made by comparison of the running pressure with the required pressure to be at a fixed point just below the MFI test pressure. The flow rate is reduced in rapid stages, until the result is achievedxe2x80x94at which point an MFI test begins. This procedure allows progress to the desired conditions of the test in the minimum time.
Discrete flow rates and pressure measurements are used, rather than the alternative method of measuring flow rate at fixed pressure. This avoids the pitfall of the constant pressure method, which is forced to have a slow response time in order to avoid oscillatory and hence inaccurate measurement of MFI.
If the constant pressure method uses alternate purge and constant pressure sections in its measurement, it will spend a longer period in the settling than the purge and dual shear rate method and hence will have a slower response time.
Constant pressure methods, for non-powder product, attempt to compensate for their inherent slow response time at low MFI values, by introducing a high flow by pass route to the measurement flow. This only goes so far and does not compete in response times of the single pump method described above.
The best figures for the two methods are 7 minutes for a bypass pump equipment mounted on an extruder. The equivalent figure for a version of the Applicants"" model P5 viscometer according to a preferred embodiment of the invention, is 3 minutes. Generally, the Applicants"" model P5 viscometer achieves 4 to 5 minutesxe2x80x94including remelting.
For pressure transducer stabilisation, a preferred embodiment of the Applicants"" (model P5) viscometer according to the invention employs:
a jacket heater under conventional closed-loop control, to stabilise the sensor and the mercury pressurisation fluid in the sensor envelope; and
a pressure transducer of the mercury filled type.
The heater stabilises two out of three effects seen by the transducer, which contribute to approximately 66% of the two variations of the devices. The remaining uncompensated fluctuation is in the stem and transfer volume of the mercury filling. Overall, the accuracy of the transducer is enhanced by factor of three.
The push rod pressure transducers which have very similar performance to mercury types can be enhanced by thermal stabilisation around the strain gauge.
Further enhancements are gained by the calibration in the region of operation against pressure standardsxe2x80x94resulting in an overall performance that is close to 0.1% accuracy, as opposed to the 1.0% of the standard device.
More sophisticated pressure transducers, with silicon encapsulation or sampling technology, obviate the need for stabilisation, because the strain gauge elements are situated in an existing temperature controlled zone.
The high throughput required by rheometers are only obtainable if a high efficiency gear box and high power motor combination are used. Refinements in gearbox and motor combinations, in preferred embodiments of the Applicants"" model P5 viscometer according to the invention include:
use of a pinion reduction gearbox and stepping motor under micro-stepping control; and
axial alignment of the motor to the gear pump by a pegged assembly that through-locates the gear box output shaft to the gear pump location dowels.
In a particular construction, drive from final gear to the gear pump is through a replaceable shaft, locating at the pump end by a barrelled hexagon drive peg, to mate with a gear pump drive plug. The gear box final pinion has an internal gear wheel, mating with a barrelled pinion on the other end of the drive shaft.
This gives such low gear box backlash and cyclic variation, that any orientation of the gear pump gives close to ideal delivery of polymerxe2x80x94which is essential for repeatable polymer flow conditions and the consequent accuracy of the rheometer.
In a preferred construction, the Applicants"" model P5 viscometer employs direct measurement of polymer temperature; thus:
the gear pump has a thermometer secured in the centre of it""s exit stream, to obtain an accurate temperature of the polymer on the way to the capillary.
The pump is modified to allow this placement. This has the advantage of a thermometer pocket, without the flow disturbance of a right-angled placement. The thermometer does not suffer side forces associated with a right angled placement, so that a narrow section sensor, 3 mm diameter, can be employed, giving faster response time and minimal intrusion to the flow. The leads of the thermometer are brought out to a convenient place with this construction.
Refinements to solids sampling handling in preferred embodiments of the Applicants"" model P5 viscometer according to the invention, include:
use of the piston itself, in conjunction with an xe2x80x98overflow weirxe2x80x99, to provide a defined sample chamber;
creating a variable sample chamber, for example by a sliding interfit of concentric cylinders, carrying an overflow chamber above them; and
provision of manual or motorised adjustment to the overflow chamber.
The adjustable sample chamber allows the purge flow rate to be maintained at a constant rate and quantityxe2x80x94which eases the clear-out problem of a rheometer running a variable purge rate to effect a constant throughput. The purge quantity can therefore be constant and the measurement quantity can be variable.
Any residue left over from these first two stages is removed with another third stage purge, which endures until the quantity of sample delivered is fully consumed.
There now follows a description of some particular embodiments, by way of example only, of principal aspects of the invention, namely:
polymer reactor control by on-line MFI measurement;
post-reactor (extruder) conversion (ie granulation or pelletisation from powder) and re-processor blending control by on-line MFI measurement; and
viscometer construction, configuration and operation for on-line MFI measurement for reactor or re-processor control;
with reference to, and as shown in, the accompanying diagrammatic and schematic drawings, in which:
In relation to (on-line) polymerisation sampling and control: